1. Field of the Invention
The present invention relates to a power control device which adjusts AC input voltage to control electric power to be applied to a load. The present invention also relates to an image forming apparatus, such as an electrophotographic apparatus, which utilizes this power control device.
2. Description of the Related Art
An electrophotographic apparatus has image forming means for forming a visual image (hereinafter referred to as a "toner image") on recording paper by means of an image developing material (hereinafter referred to as "toner"). The recording paper having a toner image formed thereon is fed to a fixing unit, in which the recording paper is passed through a nip formed by opposing heating rollers 100 as shown in FIG. 8. The heating rollers 100 use heaters H1 as shown in FIG. 7 to heat the recording paper to thereby fix the toner image to the recording paper. In this way, an image is formed on the recording paper. In fixing toner to recording paper by such a heating/fusion device, it is necessary for the surface temperature of the heating rollers of the fixing unit to be correctly controlled so as to be kept at a fixing temperature which is higher than the melting point of the toner but which does not adversely affect the recording paper.
In view of this requirement, a temperature regulation method based on a phase angle control (also referred to as "phase control") system as shown in FIG. 5 is widely used. In the following, the circuit shown in FIG. 5 and the operation thereof will be described.
When an AC voltage is applied between input terminals, an AC voltage that is insulation-divided by a transformer Ti is input to a phase angle reference signal generating circuit 2, and a phase signal at a zero cross point of AC input is output.
When a temperature adjusting reference voltage Vc is input to the input of a temperature detecting/comparing circuit 1, the temperature detecting/comparing circuit 1 reads a temperature signal from a temperature detecting device TH1, such as a thermistor, for measuring the surface temperature of the fixing rollers and compares it with the temperature adjusting reference voltage Vc, and outputs a voltage in proportion to the difference therebetween to a phase angle control circuit 3 as a control signal.
The phase angle control circuit 3 determines an energization phase angle from the value of the control signal at this time, and supplies electric current to the base of a transistor TR1 with a timing delayed with respect to the timing of the zero cross signal from the phase angle reference signal generating circuit 2 by an amount corresponding to the phase angle control. This current supply to the base of the transistor TR1 turns on the transistor TR1, whereby a photocoupler PS1 is also turned on, and a current flows to the gate of a TRIAC SCR1 to turn on the SCR1, thereby causing an AC current I.sub.AC as shown in FIG. 6 to be supplied to the heaters H1. In this way, an AC current I.sub.AC as shown in FIG. 6 is applied to the heaters H1.
When the temperature detection voltage of the temperature detecting device TH1 is lower than the temperature adjusting reference voltage Vc, the control signal is enlarged, and the energization phase angle of the phase angle control circuit 3 is enlarged to increase the power to be applied to the heaters H1, with the result that the heat generation amount increases to raise the surface temperature of the fixing rollers.
When the temperature detection voltage of the temperature detecting device TH1 is higher than the temperature adjusting reference voltage Vc, the value of the control signal is diminished, and the energization phase angle of the phase angle control circuit 3 is diminished to reduce the power to be applied to the heaters H1, with the result that the heat generation amount decreases to lower the surface temperature of the fixing rollers 100.
By the above-described operations, it is possible to control the surface temperature of the fixing rollers 100 so as to keep it at a value that is in proportion to the temperature adjusting reference voltage Vc.
In keeping the temperature of the fixing rollers or the like at a fixed temperature through AC power control, a phase angle control system using an electric power controlling device like TRIAC as shown in FIG. 5 proves itself excellent in terms of controllability. However, as shown in FIG. 6, such a power control device effects power control by cutting off part of the sinusoidal wave of an AC input power, so that, at a certain phase angle, the power control device is abruptly switched from the OFF to the ON state. Due to the voltage spike at the instant of this switching ON, the abrupt rise of the current waveform, etc., harmonic noise over a wide frequency range are generated.
These noise components include a lot of low-frequency components, so that it is impossible to completely eliminate the noise components even by using a noise filter. Thus, it constitutes one of the impediment factors to EMC (electromagnetic compatibility).
Further, the effective value of the current consumption is naturally high with respect to the power consumption, so that the power factor is also reduced, resulting in an increase in current capacity of the power source system.
To cope with this problem, an ON-OFF control, in which the ON-OFF operations of AC power are repeated on a time scale measured in seconds, has been used in temperature adjusting type heating devices using heaters or the like. This entails a problem that the power consumption amount of the apparatus when the heater in ON is much different from that when it is OFF, with the result that fluctuations are caused in the voltage of the power source system. This causes a flickering phenomenon in which fluorescent lamps, etc. flicker, which is regarded as a problem.